1. Field of the Invention
The present invention relates to an image recording apparatus and method, a method of determining density correction coefficient and a computer-readable medium therefor, and more particularly to image processing technology which is suitable for correcting density variations caused by variation in characteristics among a plurality of recording elements in a recording head.
2. Description of the Related Art
An image recording apparatus (inkjet printer) has been used which includes an inkjet type of recording head having a plurality of ink ejection ports (nozzles). In this type of image recording apparatus, problems of image quality are liable to arise due to the occurrence of density variations (density non-uniformities) in the recorded image caused by variations in the ejection characteristics of the nozzles. FIG. 19 is an illustrative diagram showing a schematic view of examples of variations in the ejection characteristics of the nozzles, and density variations appearing in recording results.
In FIG. 19, reference numeral 300 represents a line head, reference numeral 302-i (where i=1 to 8) represents a nozzle, reference numeral 304-i (i=1 to 8) represents a dot formed by a droplet ejected from the nozzle 302-i (i=1 to 8). Here, it is supposed that the recording medium, such as recording paper, is conveyed in a direction perpendicular to the breadthways direction of the line head 300 (the nozzle arrangement direction) (namely, in the direction of arrow S), and the nozzle arrangement direction in the line head 300 is taken to be the main scanning direction, while the direction of relative conveyance of the recording medium with respect to the line head 300 (the direction S) is taken to be the sub-scanning direction.
In the example shown in FIG. 19, a depositing position error occurs at the nozzle 302-3, which is third from the left (namely, the droplet ejected from the nozzle 302-3 deposits on the recording medium at a position diverging from the originally intended depositing position in the leftward direction in FIG. 19), and a droplet volume error occurs at the sixth nozzle 302-6 (namely, the droplet ejected from the nozzle 302-6 has a greater droplet volume than the originally intended volume). In this case, density non-uniformity streaks occur at the positions in the print image corresponding to the nozzles 302-3 and 302-6 producing the depositing position error and the droplet volume error (namely, the positions indicated by A and B in FIG. 19).
In the case of a serial (shuttle) scanning type of image recording apparatus, which performs image recording by driving a recording head to scan a plurality of times over the prescribed print region, it is possible to avoid density non-uniformities by means of a commonly known multi-pass printing method, but in the case of a single pass system (line head system) which records images by means of a single scanning action, it is difficult to avoid density non-uniformities.
Since it is difficult to completely prevent variations in ejection characteristics among the nozzles in terms of the process of manufacturing the recording head, then various technologies for correcting the variations have been proposed (see, Japanese Patent Application Publication Nos. 2006-212907 and 2006-347164).
With the object of eliminating stripe-shaped non-uniformities (banding) caused by a so-called “flight deflection effect”, Japanese Patent Application Publication No. 2006-212907 proposes identifying pixels where flight deflection has occurred, setting the adjacent pixels (pixels for correction) which are within a previously established distance range of the pixel suffering flight deflection, and then correcting the pixel values of these pixels for correction in accordance with the amount of flight deflection. According to Japanese Patent Application Publication No. 2006-212907, a table of correction values corresponding to flight deflection is created by establishing respective hypothetical regions between a pixel suffering flight deflection and the pixels for correction which are adjacent on either side of this pixel, calculating the pixel density in each of these regions, and then establishing correction values on the basis of the calculated pixel densities in such a manner that the density is uniform in each of the regions (see paragraphs [0129] to [0132] in Japanese Patent Application Publication No. 2006-212907).
Japanese Patent Application Publication No. 2006-347164 discloses outputting a test pattern, obtaining depositing position error data from the print results, using this depositing position error data to define a density profile D(x) which incorporates the error characteristics of respective nozzles, converting this density profile into a function T(f) by Fourier transform and then calculating a density correction coefficient by minimizing the low-frequency component of the power spectrum of this function (paragraphs [0062] to [0089] in Japanese Patent Application Publication No. 2006-347164).
However, in the technology described in Japanese Patent Application Publication No. 2006-212907, it is difficult to calculate appropriate correction values if a large number of pixels suffering flight deflection occur continuously, and problems arise in that either correction is not performed correctly, or the load involved in calculating the correction values becomes extremely great. Furthermore, if the adjacent dots are mutually overlapping, then the overlapping portion does not produce a linear density with respect to the ink volume (ink thickness) (namely, it shows non-linear characteristics), but Japanese Patent Application Publication No. 2006-212907 does not take account of the non-linear characteristics of the density in a case where the droplets deposited onto mutually adjacent pixels overlap with each other. This point applies similarly to Japanese Patent Application Publication No. 2006-347164.